The best sound quality from a given amplifier is obtained when it is matched with the impedance of the speakers that are being driven by it. The maximum power that an amplifier can provide is dependent on the impedance of the connected speakers. Typically 8 or 4 ohm speakers are connected to the amplifier output terminals providing an output signal. The signal provided to the speakers is a voltage signal and from the equations V=IR and P=IV it is clear that P=V.sup.2 /R, i.e., power supplied by an amplifier is proportional to the inverse of the resistance (or speaker impedance) and the square of the voltage.
The maximum amplifier voltage and therefore power is limited and set by the maximum rail voltage powering the amplifier transistors. In a stereo mode one speaker (channel) is connected between positive and negative rail voltages and amplifier ground. The second speaker (channel) is connected between positive and negative rail voltages and amplifier ground, but 180.degree. out of phase (known as a self inverting amplifier). In this configuration, the maximum theoretical power that can be supplied by either channel is the square of either rail voltage (difference between absolute value of rail voltage and amplifier ground) divided by the speaker impedance. In contrast, in a monaural configuration the maximum power available is dependent on the difference between the negative rail voltage and the positive rail voltage or twice the absolute value of the rail voltage--because of the phase inversion between the two stereo channels (less losses). For example, in a 100 watt per channel amplifier feeding a speaker having an impedance of 4 ohms under the formula P=V.sub.RMS.sup.2 /R, the 100 watts into 4 ohms without losses provides a RMS (root mean square) rail voltage of 20 volts RMS. The 20 volts PMS translates into 28.28 volts peak to peak between either the positive rail or the negative rail and amplifier ground. In a mono configuration the potential rail voltage is doubled thereby quadrupling the potential amplifier power if rail voltage remains constant across one 4 ohm speaker.
Some amplifier users understand that reducing the impedance of the speakers attached to an amplifier according to the equation P=V.sup.2 /R will enhance the power draw and thereby the volume available from an amplifier. For example if the amplifier in the example above had its output impedance reduced to 2 ohms per channel and the rail voltage was regulated to maintain the 28.28 volts peak to peak then a theoretical output of 200 watts per channel would be achieved.
Thus, when a user attaches a set of lower impedance speakers to the output of the amplifier the current draw and power increase proportionally. A 28.28 volt peak to peak rail voltage when the impedance is halved from 4 ohms to 2 ohms will theoretically double the power produced by the amplifier from 100 to 200 watts. Reducing the impedance still further to 1 ohm will quadruple the theoretical output to 400 watts. Reducing the impedance will cause the amplifier components to surge to maintain the desired rail voltage and test the limits of component capabilities. In reality transistors in the power supply will become overloaded and fail.
In an effort to prevent such overloads by users, amplifier manufacturers have tended to overprotect their amplifiers by providing components whose ratings are not exceeded when the speaker impedance is as low as 1 ohm. Again considering a theoretical case with no losses, a 100 watt per channel amplifier rated to supply 100 watts to a 1 ohm speaker impedance would have a rail voltage setting of 10 volts RMS or 14.14 volts peak to peak. If an ordinary user were to attach a 2, or 4, or 8 ohm impedance speakers to such an amplifier their maximum power output under such conditions would be 50, 25, or 12.5 watts, respectively. While this theoretically extreme scenario is often softened by utilizing components whose resistance increases with increasing temperature (such components providing additional impedance in series with the speaker when a 1 ohm impedance speaker is connected to the amplifier output) a large percentage of the amplifier capability remains unavailable if 8 ohm. speakers are connected to a circuit designed to tolerate a 1 ohm impedance at peak. In this scenario the user of 8 ohm speakers will be short changed by having only a fraction of the power available from the amplifier compared with the power available to a user using a 1 ohm impedance speaker.
This conservative design of amplifier circuitry results in underutilization of the amplifiers capabilities for most users. The rated power is available only under certain extreme conditions. The capability of that circuitry is underutilized and unaccessible in instances where high impedance speakers are used. Underutilization of amplifier capabilities is particularly noticeable in automobiles and other vehicles where a mobile power source provides power to the audio system. Users who use high impedance speakers find that they cannot seem to get the power from the speakers which match the amplifier rating.
Further, even at non-peak volume levels the dynamic headroom available to the music is limited because of the low rail voltage. When a low rail voltage is used with high impedance speakers, the peaks of the music signal which require voltage beyond the rail voltage are "clipped" by the limits of the voltage supplied (have insufficient dynamic headroom).
It would be desirable to utilize the full capabilities of an amplifier regardless of the impedance of the speakers attached to the amplifier so that full utilization of the components in the amplifier (less fixed and variable internal losses) can be made by all users.